Planar shadow image velocimetry for the analysis of the hydrodynamics in bubbly flows

In order to allow for more reliable modelling of microscale processes in turbulent bubbly flows, detailed experiments in a laboratory-scale double loop facility and a bubble column with a diameter of 140 mm were performed. The range of bubble mean diameters considered in the measurements was between 2 and 4 mm. The average gas volume fraction was in the range between 0.5 and 5% for different experiments. To allow simultaneous measurements of bubble size, bubble velocity and liquid velocity field, a combined system of planar shadow imaging, particle tracking velocimetry (PTV) and particle image velocimetry ( PIV) for online measurements was developed and applied. The measurements of the liquid phase velocities were realized by seeding the flow with polyamide tracer particles having a mean diameter of 65 mu m. A background illumination was established by using an array of 551 LEDs with a size of 160 mm by 100 mm. A double image CCD camera with macro optics was used to record the images of tracers and bubbles simultaneously. Because of the small depth of field of the macro camera optics (< 4 mm for the bubbles), it was possible to discriminate between bubbles and tracer particles inside and outside the camera's focal plane using the gradient of grey values. A set of digital image filters was applied to perform phase discrimination between bubbles and tracer particles, i.e. to obtain separate double images for bubbles and tracer, respectively. The contour of in-focus bubbles was determined using an edge detecting Sobel filter and a spline interpolation technique. Thereby, the bubble size, shape and orientation could be derived. The bubble velocity was obtained by applying PTV and the continuous phase velocity field was determined by PIV, using a successive refinement of the interrogation area. By recording and evaluating at least 500 double images, it was possible to determine bubble size distributions and mean as well as fluctuating velocities for both phases. Thereby, detailed data on the hydrodynamics of bubble-driven flows are provided.